What are some things that mechanical engineers know and others don’t?

What are some things that mechanical engineers know and others don't? by Niladri Sarkar

Answer by Niladri Sarkar:

The art (umm … science) of biscuit dunking.

Doesn’t the picture make your mouth water? You’ll probably be thinking of this answer on dunking the next time you have your evening tea/coffee with cookies. But one particular task you are going to serve more thought is dunking the cookie properly, making sure that the dipped segment doesn’t turn into a yucky mess and take a nose dive into the cup.

Fortunately, a guy named Edward Washburn worked on deriving the critical time that just makes sure your sweet little cookie doesn’t depart to the other side and stays with you to provide a pleasurable snack.

After many cookies went to their grave, Washburn came up with the following equation for the time required to ensure a good dunking experience by modelling the pores as a capillary flow through a bunch of parallel tiny cylindrical tubes-

Here, t is the time required, D is the diameter of the minute pores located on the cookie (of the order of 0.01 mm), L is the thickness of the cookie, η and σ are the dynamic viscosity and surface tension for the fluid (tea/coffee) at the temperature of the beverage.

Now, of course nobody will be equipped with a thermometer or a means of finding the properties of the fluid (tea/coffee) at that temperature when enjoying their snacks.

So how does this equation help?

It does when you note an important observation-

The time required for dunking varies as square of the length of the pore.

So if we can somehow decrease the length of the pores we can decrease the time required for time and a short and simple dunk into the hot beverage will suffice. We obviously cannot achieve an actual decrease in length but surely a virtual one! To do this …

Don’t dip the cookie vertically (as shown in the first pic).

Instead, dip it horizontally or slanting so that only the bottom or a small region of the cookie gets dunked and the cookie doesn’t get soaked quicker. This method of dunking ensures that a significant portion at the opposite side of the dunked surface area remains dry and the dipped part of the cookie doesn’t turn into a mushy mass or worse, collapse.

Happy dunking! 🙂

Disclaimer- The inspiration behind this write-up has been taken from the extensive work on heat transfer and fluid flow through porous media conducted by Prof. Arunn Narasimhan of the department of mechanical engineering at the Indian Institute of Technology Madras.

To my credit, I have worked on heat transfer and fluid flow through porous media and am competent enough to carry out an analysis of the Washburn equation and the nuances associated with it.